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ON THE  CHANGE  OF  REFRANGIBILITY  OF  LIGHT.
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dark, and the first is rather diffuse; they stand off a little from the others, and are a little closer together than the other four. Of the latter, the first, marked o, is very strong, considering the faintness of the light which it interrupts; the second and third are faint, and difficult to see ; the fourth, marked p, is black like the first, and a good deal broader. The line p was situated, by measurement, as far beyond H as H beyond 6. Once or twice in the height of summer, and under the most favourable circumstances, I have observed two broad dusky bands still further on. The first of these had the appearance of being resolvable into two. The excessively faint light seen beyond the second seemed to end rather abruptly at the distance represented by the border of the accompanying plate, as if there were there the edge of another dark band beyond which nothing could,be seen. In order to see the dusky bands last mentioned, and even to see the group p to most advantage, it was necessary to allow the central part of the beam incident on the prisms to pass through them close to their edges, so that evidently a great deal of light was lost by passing by the prisms altogether. This circumstance, combined with others which I have observed, convinces me that the great obstacle to seeing the fixed lines in this part of the spectrum consists in the opacity of glass. Were glass as transparent with respect to the invisible rays of very high refrangibility as it is with respect to the rays belonging to the visible spectrum, I know not how much further I might have been able to see.
I have endeavoured to identify the fixed lines in my map with the fixed lines represented in M. Silbermann's map of the chemical spectrum, with a copy of which my friend Professor Thomson has kindly furnished me. I am still uncertain respecting the identification. M. Silbermann's map is so very much more detailed than my own, and must have been made with so much purer a spectrum, that the two systems of lines are not directly comparable.
23. From the difficulty of identification some persons might be disposed to imagine that the chemical rays, and those which produced the blue light in a solution of quinine, were of a different nature, and had each a system of fixed lines of its own. For my own part, 1 was too well acquainted with the Protean character of fixed lines to regard the difficulty of identification as any valid argument in support of such a view. And that this difficulty